This invention relates to vessels for transporting cargo, and it particularly relates to improvements in vessels in which the buoyancy of the cargo is utilized to support a substantial portion of its own weight, leaving the remaining, typically minor portion to be carried by the hull. In this vessel, the hull serves to contain the cargo securely and at the same time to provide a smooth external envelope of a desired form to minimize hydrodynamic resistance; at the same time, the hull enables employment of buoyancy of the cargo by always maintaining water within the hold common with the sea, by means of permanent openings in the bottom of the hull for free passage of water in and out. The vessels of this invention may have their floating cargo loaded and unloaded into the hold by cranes, but in many instances the vessels are preferably loaded and unloaded through a gate at either or both ends. The vessels may also carry deck cargoes which are supported by the buoyancy of the vessel and excess buoyancy of any floating cargo carried.
The invention is particularly helpful in transporting cargo of the type that is shipped in barges or floating containers. A series of these barges or containers is typically moved on water by a tugboat up to a location adjacent the bow or stern of the vessel; then the series of barges or containers is loaded in through a gate in the bow or the stern. After the voyage, the same series of containers is unloaded through a gate in the bow or stern and towed away by a tugboat.
Many patents disclose vessels wherein the cargocarrying hold of the vessel is partially filled with water but only during loading and unloading. Gates in the bow, in the stern or in the sides of the vessel have been used, and various types of cargoes have been floated in and floated out. In these vessels the practice has been to pump all the water out of the hold after the cargo has been loaded, keeping the water out until the cargo is to be unloaded so that the vessel carries the full deadweight of the cargo. In all these patents, the water inside the hold is used only for floating the cargo into and out from the hold, and water is never carried in the hold during the voyage.
In contrast, U.S. Pat. No. 3,356,058 to Thomas T. Lunde discloses and claims a vessel for transporting logs. In this log-transporting vessel of Lunde, there are a ballast system and ports through the hull to achieve a flotation of part of the logs during the transport.
The present invention has the important advantage over the Lunde device that it can be used for transporting barges or containerized cargo of the type in which the containers can be floated in the water, whereas the Lunde vessel cannot.
There exist well-known systems in which barges or lighters are handled on a large high-speed, ocean-going vessel. One such system calls for towing the barges or lighters out to the vessel where they are floated over a submerged elevator which then lifts the barge .[.of.]. .Iadd.or .Iaddend.lighter out of the water to a deck level where the barge or lighter is transferred over rollers to a final stowage position. Another such system tows the lighters or barges out to the vessel where they are hoisted aboard by large cranes on board the vessel. These systems both are used so that large vessels need not be delayed in port by having to handle cargoes at a pier. They can also transport cargoes to and from small ports that otherwise could not accommodate ocean-going vessels. In many instances, this system of loading and unloading is more economical than other marine transport systems.
In seeking to accommodate such barges or lighters on vessels generally of the type of the Lunde vessel, there are problems that are not encountered in transporting logs. Logs have substantially identical specific gravities, even though they are different shapes and sizes. The cargo containers that comprise the lighters and barges are normally identical in size and shape but, due to different degrees of loading, the containers vary considerably in their specific gravities. To load them into a ship and then allow them to float freely would not be feasible. They would float at different levels, would tend to bob around, and could, under disturbed weather conditions, exert forces that could severely damage their hulls as well as the vessel's hull and might even cause the vessel to sink. Thus, there are difficult problems in making it possible to employ the buoyancy of empty or lightly filled lighters to help share the load of the heavily filled lighters with the hull. It cannot be done with the device shown in the Lunde patent. Yet, the solution of the problems gives rise to very beneficial results.
Problems relative to the loading and maintenance of the barges or lighters, as well as of other floating cargo, and problems relative to the specific situation described are also encountered.